Speakers which are categorized as "planar"-type speakers (e.g., electrostatic, ribbon or induction speakers) usually have separate panels 1, 2, and 3 for reproducing, respectively, the low , mid and high-frequency portions of the musical spectrum (FIG. 1). In some cases, the separate panels are stacked vertically, rather than laterally as in FIG. 1. Most of these laterally arranged speakers currently employ separate "line source" radiators. Exceptions to this approach ar the Quad and Beveridge electrostatic speakers. The Quad employs a system of driving an electrostatic panel through a delay network in order to produce a nearly spherical sound wavefront, thus avoiding directional projection of the sound. The Beveridge seeks to accomplish the same objective through use of an acoustic lens.
In FIGS. 1-4 of my U.S. Pat. No. 4,468,530, I disclose a planar, push-pull, induction-type speaker having a high frequency unit placed between low frequency units. The high frequency unit is long, in order to achieve acoustic output matching the low frequency unit. In the embodiment of FIGS. 5 and 6 of that patent, an acoustic lens is employed to prevent vertical beaming of the high frequency sound.